The overall objective of this work is to develop portal dosimetry by using Fast Fourier Transform (FFT) convolution dose calculations and a practical on-line portal dosimetry system to verify radiotherapy dose delivery to the target volume. Portal imaging uses the therapy beam itself to show the spatial accuracy of beam placement within the patient during treatment. The hypothesis is that portal dosimetry combined with portal imaging can be used to monitor the dose delivery sequence, increase the accuracy of beam placement, and ensure the accurate delivery of the planned dose especially in the setting of conformal and dynamic radiotherapy. The promise underlying these studies is that a properly designed portal imaging and dosimetry system will contribute to an improved therapeutic ratio. The goal of the present studies is to achieve dose deliver verification with an accuracy of 3% in the treatment volume. Three interrelated specific aims will be pursued: (1) Modification of FFT convolution calculations. Current FFT convolution calculations are limited in finding doses inside patients or phantoms. FFT techniques will be modified so that the calculation can be extended outside patients or phantoms to include the proposed portal dosimetry system. (2) Calibration of an on-line electronic portal imaging device (EPID), based on examination of the physics and the dosimetry response of the existing EPID. The goal is to develop appropriate dose response calibrations of this EPID so that it will become a practical tool for on-line portal dose imaging (PDI). (3) Quantitative treatment plan verification. With information derived from aims 1 & 2, the dynamic dose delivery sequence will be verified before the phantom is placed on the couch (pre treatment verification). Calculated portal dose distributions will be compared with the measured distributions during radiotherapy to verify planned dose delivery. Feasibility studies will be conducted with 6 MV x-ray irradiation of a modular plastic phantom and an anthropomorphic (Rando) phantom. The research results will be applied to patient studies using CT-scan data with patients at treatment positions. Pre-treatment verification and on-line checking of patient dose throughout the treatment volume is important especially in conformal radiotherapy.